Microphone sound passage structure



P 11, 1951 G. L. CARRINGTON 2,567,875

MICROPHONE SOUND PASSAGE STRUCTURE Filed Feb. 24, 1950 2 Sheets-Sheet 1 @EOQGE 1:. CAER/NGTON,

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Sept. 11, 1951 G. L. CARRINGTON MICROPHONE SOUND PASSAGE STRUCTURE Filed Feb 24, 1950 2 Sheets-Sheet 2 INVENTOR. GEORGE L. 6422mm 701v,

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%IMOW@ Patented Sept. 11, 1951 UNITED STATES PATENT OFFICE MICROPHONE SOUND PASSAGE STRUCTURE George L. Carrington, Encino, Calif.

Application February 24, 1950, Serial No. 146,075

5 Claims. (01. 179-111) This invention is concerned generally with microphones for translating sound waves into corresponding electrical oscillations, as in amplifying, recording, or measuring speech, music and other sounds.

For such purposes it is desirable that a microphone have a fiat frequency response, that is, that the amplitude of the resulting electrical oscillations bear a uniform ratio to that of the sound waves over a wide range of frequencies.

Further, it is especially desirable that a microphone be non-directional, that is, that the amplitude of the resulting electrical oscillations be independent of the physical orientation of the microphone with respect tothe direction of travel of the sound waves in the air or other medium.

A primary object of the present invention is -to provide a microphone that is substantially in the sound transmitting medium take place so slowly at those lower freqencies that there is ample time for the pressure to become effectively equalized at all points on the surface of the microphone case. However, at higher frequencies the wavelength becomes comparable to the dimensions of the microphone case of usual size. The sound pressures, at any instant, may be quite difierent at different points on the case; and the phase of the sound pressure, more particularly, the peak value or amplitude of the periodic pressure change at that side of the case from which the sound is coming is typically quite different from the amplitude at the opposite side. Moreover, the sound amplitude may vary relatively rapidly and irregularly over the case surface, so that the amplitude even at two neighboring points may be appreciably different.

It is therefore not surprising that serious difficulty has been experienced in providing a microphone that maintains a non-directional characteristic not only at relatively low frequencies, but also throughout the range of frequencies up to and including those for which the wavelength is comparable with the case dimensions.

The present invention is especially useful in connection with microphones of any type that utilizes a vibratory diaphragm housed in a case that includes an enclosing side wall transverse with respect to the plane of the diaphragm and an end closure wall generally parallel to that plane, the case being so constructed as to expose one face of the diaphragm to the pressure variations associated with the sound waves. In such a microphone, the pressure difierence at opposite faces of the diaphragm causes it to vibrate in accordance with the sound pressure to which it is exposed. The vibratory movement of the diaphragm may then be translated into electrical oscillations in any one of several well-known manners. Preferably, but not as a necessary limitation on the invention, a fixed plate may be provided in closely spaced parallel relation to the diaphragm, in such a way as to form an electrical condenser of which the capacity varies in accordance with the vibratory position of the diaphragm. By connecting that condenser in a suitable electrical circuit, the variation in capacity may be translated into a variation in voltage, and the latter may be amplified and utilized in any suitable manner.

It is common practice in microphones utilizing such diaphragms (whether operating on the condenser principal or otherwise) for the diaphragm to be located just inside of and in par,

allel spaced relation to the end closure wall of the case. One or more apertures are ordinarily provided, distributed either centrally or more or less uniformly in that end wall of the case, offering a low resistance fluid connection between the air or other medium outside the case and the sound chamber formed between the diaphragm and the case end wall. Fluid flow through that fluid connection causes the pressure within the sound chamber, and in contact with one face of the diaphragm to vary in accordance with the varying sound pressure just outside the end wall of the case.

it is well known that the arrangement just .de-

scribed has the disadvantage that the frequency response of the microphone depends in general upon the direction of propagation of the sound. A typical result is that the microphone is reasonably non-directional at lower frequencies, but that the response to higher frequencies is greater for sound directed normally to the diaphragm and to the end wall of the case than for other relative orientations. That means that the microphone cannot give a flat response in all positions. Either th higher frequencies are enhanced for sounds travelling parallel to the microphone axis, or the higher frequencies are attenuated for sounds travelling transversely with respect to that axis.

Furthermore, the usual type of case, such as has been described, does not protect the diaphragm from physical damage as effectively as would be desirable. Although the use of sufficiently small sound apertures in the case wall offers some protection, a relatively sharp instrument can readily be inserted into the diaphragm chamber whether intentionally or not, with a high probability of injuring or breaking the diaphragm.

In accordance with the present invention, the sound passages through the microphone case are not located centrally in the end wall of the case, as in previous microphones, but are" so located that they open through the outer surface of the case close to the edge that surrounds that end wall and defines it from the case side wall. In the preferred embodiment of the invention, the sound passages open externally throu h the side wall of the case, near the edge just referred to, and the case one wall is externally imperforate. Moreover, the inner mouths of the sound passages open into the sound chamber, not opposite the central portion of the diaphragm", as in previous practice, but at eircumferentially' s aced positions Opposite the eripheral portion of the diaphragm.

The whole microphone assembly, including its case is also preferably very small, so that the diffioulties' which are encountered with sound waves of lengths com arable with the microphone dimensions are minimized.

A full understanding of the invention and its further objects and advantages will be had from the following description of a particular preferred manner of carrying it out. That description, and

the appended drawings, which form a part or it, are illustrative, and are not intended as a limitation upon the scope or the invention.

In the drawings,

Fig. 1 is an elevation of a typical microphone of condenser type embodying the invention;

Fig. 2 is a vertical section taken in the same aspect as Fig, 1, and on line 2=-2 of Fig. 3;

Fig. 3 is a plan view of the microphone of Fig. 1;

Figs. 4 and 5 are horizontal sections taken on lines 4-4 and 5-5, res ectively ofFig. 2; and

Fig. 6 is an exploded perspective.

Fig. 2 of the drawings shows the present preferred practical embodiment or the invention at a scale of about eight times. In the preferred modification illustratively shown in the figures, the microphone case is indicated generally by the numeral l5 and comprises primarily a side wall l6, ill-ustratively shown as of cylindrical form, and

an end closure wall I? on the upper end of the cylinder and preferabl integral with it. Side wall it and end wall 11 meet at the edge H), which is preferably rounded off either quite sharply or relatively gently. (Terms such as upper and lower, here and elsewhere in the present specification and claims are not limitativ'e, but are for convenience of description only, and refer to the typical illustrated position of the microphone.) External threads [8 on cylindrical side wall It facilitatemounting of the microphone on a suitable support-,- indicated at 22, while internal threads serve for securing the various internal elements of the microphone structure in position within'the case.-

Diaphragm 25 preferably forms the vibratory plate of a condenser microphone, and, as such, it is preferably formed of a thin wafer of glass or quartz with a thin layer or film of conductive material on its upper face-its face which lies away from the fixed condenser plate, here shown as formed by the face 33 of a stud 34. The electrically effective area of the diaphragm is preferably limited to substantially its central portion' either by limitin the coated area on its upper face, or, as shown here, by limiting the area of plate surface 33 to the central diaphragm area. The case preferably has an internal peripheral shoulder formation which presents a downwardly facing shoulder 29 at a level preferably below the under or inner face 52 of end closure wall ll. That shoulder formation may, as hereinafter pointed out, be regarded as a pcripheral thickening of either the end closure wall or of the side wall. The sound entry passages 50', as" set out later, preferably enter laterally through the case wall at a level above that shoulder; and the shoulder also functions as an element in the internal structure for supporting the diaphragm and other internal parts of the microphone. Insofar as my invention is concerned the structure for supporting the diaphragm in its described position in the case (and for supporting the fixed plate electrode 34 of the preferred condenser type microphone in parallel spaced relation to the diaphragm) may be any suitable structure. The structure shown here (subject matter of the application of William J. Moreland, Jr'., Ser; No. 146,133, filed February 24, 1950) is suitable and will be explained only in general.

Diaphragm 25 is peripherally clamped between the upper annular face 28 of a mounting ring 30 and a washer 26 which is pressed down by an annular spring 21 lying under case shoulder 29. Mounting ring 30 is threaded into the case against a spacer ring 32.

Electrode stud 34 is mounted in an insulating disk' 36 which is carried in an externally threaded ring 35. Ring 35 is threaded into a depending skirt 38 of mounting ring 30. Spacing adjustment of electrode face 33 with reference to the diaphragm is done by turning ring 35 in and out on its threads, and the ring and'electrode 34 are locked in adjusted position by compressing skirt 38. For that purpose an internally tapered clamping ring 46 is pressed into place by a lock ring 42 threaded into the case. A thin annular washer 43 has peripherally spaced ears 44 which provide for localized pressure of clamp ring 4|] on the skirt 3B. The described structure holds the diaphragm very accurately and firmly in its position in the case, and provides for very accurate adjustment of the fixed electrode spacing and maintenance of its parallelism to the diaphragin;

Many alternative systems are known for transforming the vibratory movement of a diaphragm such as 25 into electrical oscillations. For example, the diaphragm 25 may be made of, or coated either all over or in its central portion only with, magnetic material, and the head of stud 34 may represent a horizontal coil of fine wire in which the current is variable in accordance-with changes in the position of the diaphragm.

The diaphragm movement is caused by the varying pressure difference between the relatively closed chamber 48 immediately below the diaphragm and the relatively open sound chamber 49 immediately above the diaphragm. Chamber 48 is preferably not hermetically sealed, but

its walls, comprising primarily" diaphragm 25, mounting ring 30 and the fixed condenser plate assembly '34, 35, 36-,-are sufficiently tight that-the pressure inside the chamber responds relatively slowlyto pressure changes outside. The volume of chamber 48 is sufliciently large with relation to the maximum movement of diaphragm 25 that the chamber pressure is modified only very slightly by such movement; and at the same time the chamber is small enough that no appreciable resonance takes place.

Sound chamber 49 is in direct communication with the space surrounding case l via the sound transmitting passages 50. In accordance with the present invention, those passages are so formed that they pierce the outer surface of case [5 at points distributed about the periphery of the case, close to or substantially at the edge l9. In the'preferred embodiment illustrated, passages 50 are formed by cutting 'threestraight slots in a plane normal to the microphone axis and at 120 intervals about the periphery of the microphone. Those slots pass through wall [6 above annular shoulder 29, so that they do not break the continuity of that shoulder. As shown, the level of the lower faces 5| of cuts 50 is at or slightly below the inner face 52 of case end wall l1. Alternatively, that feature of the construction may be considered to constitute a downward offsetting of the central portion of the inner face 52 of end wall IT with relation to the peripheral portion of the same face; the peripheral portion being formed by upper faces 51 of the cut slots 50. That peripheral portion of the end wall is thus spaced further from diaphragm 25 than is the central portion 52, and the sound passages open into the sound chamber (considered from the present viewpoint) primarily above the plane of the central portion of the end wall.

An advantage of that structure is that the volume of sound chamber 49 is kept relatively small with respect to the cross-sectional area of sound passages 50, facilitating the transmission of sound pressure variations to the chamber. Each sound passage may be characterized as comprising a slot portion 54, piercing cylindrical wall IS in a plane parallel to diaphragm 25, and a channel portion 55, cut in the inner face of end wall I! and opening generally normally to the diaphragm into sound chamber 49. As seen in Figs. 4 and 5, the channel portions of the passages 50 are segmental in shape, the straight bottom surfaces 53 of the cut slots forming the chords of the segments. As will be obvious from the drawings and the above description, the slots may have curved, rather than straight bottom faces 53, with the result that channel portions 55 are lunar rather than segmental.

If the alternative view is taken that annular shoulder 29 comprises a portion of case end wall ll, then the passages 50 may be considered to lie entirely within the circular end wall of the case, opening radially to the outside close to edge l9, and opening transversely into sound chamber 49 through the chamber wall surface 52 that is directly opposite diaphragm 25. The openings into chamber 49 are located close to the clamped edge portion of the diaphragm, and are spaced radially outwardly from the central portion of diaphragm 25 that is directly opposite fixed condenser plate face 33 and is alone directly effective in producing electrical oscillations.

That central active portion of the diaphragm is opposite the parallel and imperforate central portion of inner face 52 of case end wall [1.

The structure exhibits a four-fold parallelism; comprising wall surface 52, diaphragm 25,- fixed condenser surface 33 and,=in' particular, the com-.- mon plane of the plurality of sound passageslots 50. Thus the sound isled into sound chamber 49 first along apath within the slots 50 substan tially parallel to the case end-wall and-to the diaphragm, and then along a shorter path sub stantially at right angles to the plane of the slots, and to the-diaphragm. The variations in sound pressure are thus directed primarily normally against that annular portion of the diaphragm that immediately surrounds its central and active portion.

As is seen best in Fig. 2, the preferred embodiment of the invention provides remarkably good protection of the diaphragm from physical damage. The passages 50 include a right angle bend between the slot portion 54 and the internal opening of channel portion 55. Thus no straight tool or instrument can be inserted through such a passage into contact with the diaphragm.

The preferred embodiment that has been described is intended to be only illustrative of the invention, which can be embodied also in microphones of many different types and styles. The manner of construction may be varied in many respects without departing from the proper scope of the invention, which is defined in the following claims.

I claim:

1. In a microphone Of the type which includes a. vibratory diaphragm and case wall structure forming an enclosing side wall and an end closure wall, the diaphragm being peripherally supported within the case in spaced and generally parallel relation to the end wall and forming therewith a sound chamber, the case wall being formed with sound apertures which permit fluid flow between the said sound chamber and the region surrounding the case; the improvement which is characterized by the fact that the end closure wall is substantially flat, is spaced from the diaphragm by a distance that is small compared to the diameter of the diaphragm, and forms with the side wall a well-defined external case edge, all of the said sound apertures piercing the exterior surface of the case wall in the immediate vicinity of the intersection of the side wall and the end Wall.

2. Microphone improvements as defined in claim 1 and in which the sound apertures are located in the side wall near the end wall, and the effective area of the diaphragm is substantially limited to its central portion.

3. In a microphone of the type which includes a vibratory diaphragm and case wall structure forming an enclosing side wall and an end 010- sure wall, the diaphragm being peripherally supported within the case in spaced and generally parallel relation to the end wall and forming therewith a sound chamber, the case wall being formed with sound apertures which permit fluid fiow between the said sound chamber and the region surrounding the case; the improvement which is characterized by the fact that the inner face of the end wall includes a central portion spaced relatively close to the diaphragm, and an outer portion spaced farther from the diaphragm than the central portion, and that the said sound apertures open through the side wall of the case and lie substantially between the planes of the said central and outer portions of the inner face of the end wall.

4. In a microphone of the type which includes structure forming a generally cyllndrical'case' having an end closure wall at one end, and a peripherally supported vibratory diaphragm within the case in spaced parallel relation to the inner face of the end wall and forming therewith a; sound chamber, the case wall being formed with sound apertures which permit fluid flow between the region surrounding the case and the said sound chamber; the improvement which is characterized by the fact that the end; wall of the case is substantially fiat, is spaced from the diaphragm by a distance that is small compared to the diameter Of the diaphragm, and forms with the cylindrical case a well-defined external ease edge, all of the said sound apertures opening outwardly in a generally radial direction through the cylindrical portion of the exterior case surface close to the said edge, and opening inwardly in a generally axial direction into the sound chamber through the peripheral portion of 8 the relatively fiat interior surface of the end wall.

5. Microphone improvements as defined in claim 4 and in which the efiective area of the diaphragm is substantially limited to its central portion, radially inward of the inner ends of the sound apertures.

GEORGE L. CARRINGTQN.

REFERENCES CITED The following references are of record in the file of this" patent:

UNITED STATES PATENTS Number 7 Name Date 1,540,506 Watkins June 2, 19:25 1,599,682 Sherrod Sept. 14, 1926 1,934,416 Flanders Nov. '7', 1933 2,086,107 Wilson July 6, 1937 2,490,563 Van Gastle Dec. 6, 1949 

